1. Field of the Invention
The present invention relates to a zoom lens suitable for an imaging apparatus equipped with a solid state image sensor, such as digital still camera and a digital video camera.
2. Description of the Related Art
Zoom lenses that are configured by 4 lens groups, capable of high zoom ratios, and used as imaging lens on digital still cameras and digital video cameras equipped with a solid state image sensor such as such as a charge-coupled device (CCD) and a complementary metal-oxide-semiconductor (CMOS) are commonly known (see, for example, Japanese Patent No. 4672860).
The zoom lens recited in Japanese Patent No. 4672860 includes sequentially from a side nearest an object (object side), first to fourth lens groups respectively having a positive, negative, positive, and positive refractive powers. In the zoom lens, the first and third lens groups are stationary, while the second lens group is moved in one direction to perform zooming and the fourth lens group is moved in a back and forth direction to correct image plane variation accompanying zooming and to perform focusing. The zoom ratio of the zoom lens is on the order of 25×; and the maximum angle of view is on the order of 58 degrees. With such a zoom lens formed by 4 lens groups, since there are 2 movable groups, configuration of the lens barrel can be simplified and the size of the lens system overall can be reduced.
A zoom lens that incorporates a stationary fifth lens group into the 4 lens group configuration above is further known (see, for example, Japanese Patent Nos. 4542933, 4823680, and 4823684). The zoom lenses recited in Japanese Patent Nos. 4542933, 4823680, and 4823684 each include from the object side, first to fifth lens groups respectively having a positive, negative, positive, positive, and negative refractive power. In the zoom lenses, the first, third, and fifth lens groups are stationary, while the second lens group is moved in one direction to perform zooming and the fourth lens group is moved in a back and forth direction to correct image plane variation accompanying zooming and to perform focusing.
The zoom ratio of the zoom lens recited in Japanese Patent No. 4542933 is on the order of 30×; and the maximum angle of view is on the order of 60 degrees. The zoom ratio of the zoom lens recited in Japanese Patent No. 4823680 is on the order of 35×; and the maximum angle of view is on the order of 73 degrees. The zoom ratio of the zoom lens recited in Japanese Patent No. 4823684 is on the order of 14×; and the maximum angle of view is on the order of 70 degrees.
Nonetheless, in addition to favorably correcting various types of aberration over the entire zoom range, large aperture ratios for wide angle views enabling the recording of images at dimly lit locations over a wider range are demanded of lens systems for surveillance cameras. Furthermore, accompanying the increased prevalence of solid state image sensors capable of full high vision image recording, lens systems compatible with full high vision solid state image sensors are demanded. Conventionally, demand has increased for a lens system for a surveillance camera equipped with a full high vision solid state image sensor to have extremely high optical performance capable of favorably correcting various types of aberration over the entire zoom range.
As conventional technologies, the zoom lenses above have difficulty in maintaining high optical performance in addition to achieving greater aperture ratios and a compact size. In other words, problems arise in attempting to realize wide angle views and greater aperture ratios, including a sacrifice of compactness, difficulty in correcting various types of aberration over the entire zoom range, and an inability to record full high vision images.
For example, with the conventional technologies, if the angle of view is to be increased, the front lens diameter has to be increased and the correction of coma and astigmatism at the wide angle edge becomes difficult. Furthermore, if the zoom ratio and resolution are to be increased, the overall length of the lens system increases or chromatic aberration at the telephoto edge becomes prominent and optical performance markedly deteriorates. In particular, if greater aperture ratios are to be realized, the correction of image plane curvature from the wide angle edge to the telephoto edge becomes difficult and high performance over the entire zoom range cannot be maintained.